1. Technical Field of the Invention
The present invention relates generally to methods of manufacturing spark plugs for use in internal combustion engines of, for example, motor vehicles and cogeneration systems. More particularly, the invention relates to a method of manufacturing a spark plug for an internal combustion engine, which ensures an accurate and effective adjustment of a spark gap in the spark plug.
2. Description of the Related Art
In manufacturing a spark plug for an internal combustion engine, it is necessary to adjust the spark gap between a pair of center and ground electrodes of the spark plug. Further, to adjust the spark gap, the ground electrode is generally pressed and bent toward the center electrode. Since the ground electrode generally springs back after being pressed, it is necessary to perform the press while taking into account the amount of springback of the ground electrode. However, the amount of springback of the ground electrode is unique to each spark plug. In other words, the amounts of springback of ground electrodes vary among individual spark plugs. Therefore, if the amount of springback was not suitably taken into account, it would be difficult to define a desired spark gap for each individual spark plug.
Japanese Patent First Publication No. 2000-164322 discloses a method of manufacturing a spark plug, according to which: the amount of springback of the ground electrode to be made by a regular press is first estimated based on the amount of springback of the ground electrode made by a test press; then the regular press is performed to bend the ground electrode by an amount that is determined based on the estimated amount of spring back of the ground electrode. However, with this method, since there is no fixed relationship between the amounts of springback of the ground electrode made by the test and regular presses, the actual amount of springback of the ground electrode made by the regular press does not always agree with the estimated amount.
Japanese Patent First Publication No. H11-121144 discloses a method of manufacturing a spark plug, according to which: preliminary strikes are first made against the ground electrode; the reduction in the spark gap by the preliminary strikes are measured; the number and/or force of finishing strikes required to bring the size of the spark gap into agreement with a target valve are determined based on the reduction in the spark gap by the preliminary strikes; then the finishing strikes are made in accordance with the determined number and/or force of finishing strikes. However, with this method, since there is no fixed relationship between the reductions in the spark gap by a preliminary and a finishing strike, the size of the spark gap cannot always be brought into agreement with the target value.
Japanese Patent First Publication No. H3-64882 discloses a device for forming a spark gap in a spark plug, which repeatedly presses the ground electrode while measuring the spark gap. With this device, however, the ground electrode is pressed, each press stroke by a fixed amount that is determined by taking into account the amount of springback of the ground electrode, until the size of the spark gap is decreased below a target value. Therefore, when the amount of springback of the ground electrode varies among individual spark plugs, the spark gap may be formed too small in some spark plugs.
Japanese Patent First Publication No. H8-153566, an English equivalent of which is U.S. Pat. No. 5,741,963, discloses an adjustor for adjusting a spark gap in a spark plug. The adjustor bends the ground electrode by means of a hammering device so as to minimize the amount of springback of the ground electrode. However, with this adjustor, the number of hammerings is determined based only on the size of the spark gap measured prior to the adjustment. Consequently, when the amount of springback of the ground electrode varies among individual spark plugs, the spark gap also varies among the individual spark plugs.